Historically when man in his vocation or avocation has endeavored to determine the size of an object with greater precision than that gained from "eye-balling", he has generally had two choices: He can use a mechanical means such as the well-known rule or tape measure or he can use an electronic apparatus employing electromagnetic or ultrasonic waves.
Mechanical means such as rules and tape measures are portable and simple to use, but suffer severe limitations as to accuracy and range of measurement. Everyone is familiar with the difficulty of obtaining an accurate distance or length measurement using a tape measure: The opposite end of the tape must be held by someone else, it must be in a straight line, it must be exactly horizontal or vertical, there must be no kinks in the tape, and so on. Generally, for measurements greater than a yard, it is a two-man operation with its attendant communication problems, and the results are usually not very accurate nor quickly obtained. Furthermore, there is always the danger of damaging the object being measured by contact with the measuring device.
Electronic measuring devices, on the other hand, have provided greater accuracy and avoided damage to the object measured. These advantages, however, have generally been achieved at costs of operational complexity, cumbersomeness, and high price.
The concept of using electromagnetic and sound waves to measure distance is well-known in the art. Various generally highly accurate methods have been devised, among which are phase comparisons of the transmitted and reflected waves (Hildebrand, U.S. Pat. No. 2,956,472), double image overlap (Honeywell system, U.S. Pat. No. 3,274,914), ultrasound time-lag (Japanese Patent Publication No. 47-048,408), and intensity comparison (Takahashi. U.S. Pat. No. 4,518,253). Much of the prior art has been directed to specific uses. For example. Dewar's U.S. Pat. No. 4,541,721 is an apparatus for checking the gap between hood and fender in automobile manufacture. the Honeywell and Takahashi systems are for cameras, and Niss' U.S. Pat. No. 3,765,764 is a coordinate measuring instrument for scientific and engineering uses.
Thus, the prior art in electronic measuring devices has emphasized extreme accuracy and specialized uses. Generally the devices are complicated, requiring specialized skill and procedures. No one has considered using these highly accurate methods in a convenient, low-cost device to provide the measurement needs of business and home.